A very important aspect of hydrological measurement practice is continuous monitoring of the water table, since this is needed for resolving environmental problems caused by manmade developments.
Water occurs naturally in the form of surface water and ground water. In the surface water category are rivers, streams, lakes and oceans. The water level of surface water is normally easily accessible which makes installation of equipment for measuring and recording this level relatively easy.
By contrast boreholes have to be sunk in order to monitor ground water feeds, and these are normally constructed using 2" to 6" pipes. These pipes are normally capped to prevent unauthorised access.
The requirements of a ground water measuring system are very exacting because of local environmental conditions and specifications imposed by measurement network operators. Some of these requirements are listed below.
Total installation inside 2" to 6" pipes PA1 Operating temperature -20.degree. C. to .+-.60.degree. C. PA1 Resistance to water condensate PA1 Stable long-term measurement accuracy of +1 cm over 10 m measurement run PA1 Independent power supply for at least 1 year PA1 Low acquisition costs PA1 Low operating costs PA1 the principle of the introduction of air bubbles can be applied; PA1 a two inch pipe can be used to house the equipment; PA1 reliable operation is assured at outside temperatures of -20.degree. C. to +60.degree. C.; PA1 resistance to water condensate is assured; PA1 stable long-term measurement accuracy is guaranteed of +/-1 cm over 10 m; PA1 a self-contained power supply is provided for at least 1 year, something which has been inconceivable for other existing systems; PA1 compared with other existing systems, the procurement cost of this equipment is up to 50% lower, and operating costs are low.
Nowadays a large number of measurement systems are available for measuring water levels. But in the light of the overall requirements outlined above not all measurement procedures are suitable for measuring ground water in conjunction with electronic data collectors.
Measurement procedures so far invented can basically be placed in two categories, those measuring length and those measuring pressure. Those measuring length include all measurement systems operated by means of floats with angle encoders, acoustical and optical procedures and measurement using conductivity levels. Those measuring pressure include all systems such as pneumatic pressure recording (introduction of air bubbles) and electronic pressure measurement (piezo resistant, capacitative etc.)
For the purposes of ground water measurement the only systems to have proven themselves in practice are measurement of length by means of floats with angle encoders and electronic pressure measurement. There is a DVWK paper, Issue 107: Ground Water Measurement Devices, Bonn, 1994 which provides a current overview of the use of ground water measurement systems.